II STRUCTURE—RENAIL AND REPRODUCTIVE ORGANS 55 
the vesicle, where they form one or more concentrically laminated 
concretions of a yellowish or brownish colour, sometimes coated 
with a chalky deposit. These concretions contain uric acid, and 
in a large Ascidian are very numerous. The nitrogenous waste 
products are thus deposited and stored up in the renal vesicles in 
place of being excreted from the body. In other Ascidians the 
renal organs may differ from the above in position and structure: 
but in no case have they any excretory duct, unless the neural 
gland is to be regarded as one of the renal organs—which has not 
yet been proved. 
Reproductive Organs.—Ascidia mentula is hermaphrodite, 
and the reproductive organs le with the alimentary canal, on the 
left side of the body (Fig. 19, ov). The ovary is a ramified gland 
which occupies the greater part of the intestinal loop. It con- 
tains a cavity which, along with the cavities of the testis, is derived 
from an embryonic coelom; the ova are formed from its walls, 
and fall when mature into the cavity. The oviduct is continuous 
with the cavity of the ovary, and leads forward alongside the 
rectum, finally opening near the anus into the peribranchial 
cavity (Fig. 18, 9.d). The testis is composed of a great number 
of delicate, branched tubules, which ramify over the ovary and 
the adjacent parts of the intestinal wall. These tubules terminate 
in ovate swellings. Near the commencement of the rectum the 
larger tubules unite to form the vas deferens, a tube of consider- 
able size, which runs forward alongside the rectum, and, like the 
oviduct, terminates by opening into the peribranchial cavity close 
to the anus. The lumen of the tubules of the testis, like the 
cavity of the ovary, is a part of the embryonic mesoblastic space, 
and the spermatozoa are formed from the cells lining the wall. In 
some Ascidians (certain Molgulidae and Cynthiidae), reproductive 
organs are present on both sides of the body, and in others, 
as in Polycarpa, there are many complete sets of both male and 
female systems attached to the inner surface of the mantle on 
both sides of the body and projecting into the peribranchial 
cavity. 
Embryology and Life-History of a Typical Ascidian. 
The eggs of Tunicata are for the most part of small size, 
nearly colourless and transparent, and with little or no food-yolk. 
